They have taken from nature, but they are thieves because everything in nature belongs to Krsna. Isavasyam idam sarvam: "Everything is God's creation." (Iso, mantra 1)
In Bhagavad-gita Krsna states that if one does not perform yajna [sacrifice], he is a thief. Yajna means acknowledging that things have been taken from Krsna.
We should think, "Krsna, You have given us many, many things for our maintenance." This much acknowledgement Krsna wants; that's all. Otherwise, what can He expect from you? What are you in His presence?
We should acknowledge Krsna's kindness. Therefore, before we eat we offer the food to Krsna and say, "Krsna, You have given us this nice food, so first You taste it." Then we eat it.
- Srila Prabhupada